Stenström became an enthusiast of Tolkien in 1967. Since 1976, when he became a member of the the Tolkien Society Forodrim, he carries the byname Beregond.

Stenström has researched different aspects of the works of Tolkien, published several articles, and been a speaker at and organizer of conferences and conventions. Together with Kristina Valinger, Stenström has been responsible for the Swedish subtitle creation of Peter Jackson's The Lord of the Rings. He also contributed with help on facts and nomenclature to translator Erik Andersson for the work on the new Swedish translation of The Lord of the Rings.

Stenström is currently one of the organizers of Omentielva, the International Conference on J.R.R. Tolkien's Invented Languages, a series of biennial conferences at changing locations. He also editor for the journal Arda.